1. Field of the Invention.
The present invention relates to a method of fabricating a P-channel device having a higher breakdown voltage without increasing device size or requiring additional fabrication steps, and, more particularly, to implanting P type dopant during the P-field implant step into regions of the silicon expected to lie at the silicon-silicon dioxide interface, thereby compensating for later accumulation of N type dopant at the silicon-silicon dioxide interface due to segregation.
2. Description of the Related Art.
P-channel devices are used extensively in row decoders of non-volatile memory devices having ever-shrinking geometries. Because of the corresponding increase in substrate dopant concentration required to accommodate the shrink in lateral distances in these devices, P-channel devices are not able to withstand the voltage requirements of these row decoders.
FIG. 1 shows two current paths through P-channel device 100. Proper, desired current flow indicated by solid line 102, is from source contact 104 to source 106, through channel 108 to drain 110 and out through drain contact 112. Unwanted drain-substrate (DSS) breakdown current flow indicated by dashed line 114, is from source contact 104, to source 106, through channel 108 to drain 110, and then directly into substrate 116 and out substrate contact 118.
One of the primary reasons for reduced breakdown voltages in modern P-channel devices is the higher concentration of P type dopant at key regions of the device. Referring again to FIG. 1, field oxide 120 electrically isolates drain 110 from other electrically-active regions of the P-channel device. However, high concentrations of N dopant that accumulate at silicon-silicon dioxide interface 122 can facilitate current flow from drain 110 into substrate 116. This relationship between elevated dopant concentration and current flow is known as "avalanche breakdown", and has been described at length in Physics and Technology of Semiconductor Devices, Andy S. Grove, John Wiley and Sons, Inc., New York, 1967.
Unfortunately, accumulation of N type dopant at the silicon-silicon dioxide interface is inevitable during the growth of silicon dioxide because of the natural process of segregation. FIG. 2 illustrates the process of segregation. FIG. 2A shows a layer of N doped silicon 200 having a surface 201 at height 202. As shown in FIG. 2B, upon exposure of surface 201 of silicon 200 to oxidizing conditions, surface 201 reacts with oxygen to create silicon dioxide 203. Formation of silicon dioxide 203 out of silicon 200 and oxygen causes the surface to swell from the original height 202. N type impurities originally present in silicon 200 are excluded from silicon dioxide 203, and are "segregated" and accumulate in the silicon 200 along the silicon-silicon dioxide interface 204.